Radio Frequency Identification (RFID) is a method of identifying unique devices, and information contained therein, using radio waves. Typically, a reader communicates with a tag/device which holds unique digital information in a microchip. The tag/device may employ select materials to reflect back a portion of the transmitted radio waves thereby providing an indication that the tag/device is within the field or zone of the interrogating reader.
RFID systems can be passive, active or semi-passive. A passive RFID system employs a passive tag/device having a receiving antenna which is sufficiently large, or selectively shaped/dimensioned, to capture (or harvest) energy from a surrounding electromagnetic field, i.e., the radio frequency energy, to power the passive tag/device. The receiving antenna is electrically connected to a central microchip which performs various preprogrammed RFID functions. An active RFID system employs an active tag/device having an energy source, e.g., an embedded battery, to power the active device and broadcast signals to the reader. Consequently, active RFID systems do not require an electromagnetic field to power the tag/device. A semi-passive system is similar to active systems in the sense that an energy source is employed to power a tag/device, however, the energy is used to activate or augment the microchip's processing capability rather than to broadcast signals back to the reader. The tags/device employed in semi-passive systems are also referred to as battery-assisted tags inasmuch as a portion of the energy is acquired from the tag while another portion is obtained from the read field of the antenna. As such, semi-passive tags increase the read range of the RFID system.
RFID systems operate in the Low, High and Ultra-High Frequency radio frequency bands or between the range of about 125 kHz (LF) to about 960 MHz (UHF). Generally, RFID devices operating in the LF bands are preferable for short ranges while those operating in the high or ultra-high frequency (HF or UHF) bands are preferable for longer ranges. RFID technology is continuously improving and, currently, readers can produce a field such that a device located therein can be energized and/or interrogated at distances of up to 1000 feet (especially when using powered/active tags).
Readers often employ multiplexers which utilize a plurality of antennas activated serially or in seriatim. More specifically, each of the antennas interrogates a field or zone for a certain period of time to ensure that all tags, i.e., anticipated to be located within the particular read field, have had sufficient time to harvest the RF energy and report its information back to the reader. For example, a reader having eight (8) antennas each interrogating a field for a period of about ten (10) seconds (i.e., to ensure that each tag/device within a specific read field has had ample opportunity to respond/report information) will require a total of eighty (80) seconds (1⅓ minutes) to complete one full interrogation cycle. Therefore, as the number of antennas increase to cover a larger territory, the time required to complete an interrogation cycle increases proportionally.
RFID systems facilitate tracking of various items (i.e., items which have an accompanying RFID tag/device) for the purpose of controlling merchandise, maintaining inventory levels, or simply monitoring the location and/or flow of important documents. Furthermore, inasmuch as RFID systems utilize time-stamping (typically measured from a common reference such as the EPOCH) to identify when a particular tag/device was identified by the reader, certain information can be ascertained and conclusions drawn from information reported.
For example, if eight (8) tagged items report their presence within one read field, and, following a subsequent interrogation cycle, six (6) tagged items report their original location/read field with the remaining two (2) tagged items reporting a location in an adjacent read field, then one may conclude that two (2) of the tagged items have moved from one location to another during the elapsed time period. If, on the other hand, four (4) tagged items moved from their original location/read field, but two (2) of the tagged items returned to their original location/read field within the time frame of an interrogation cycle, then the RFID system may incorrectly characterize the transfer or movement of all of the items. That is, two (2) of the tagged items may incorrectly be characterized as having been stationary, i.e., remaining in the original location, for the duration of the interrogation cycle—despite the fact that these two items moved and returned before the system could properly or correctly track such movement.
The example above, therefore, highlights the difficulties or inaccuracies caused by the time elapsed during an interrogation cycle. More specifically, it emphasizes the need for minimizing the time required for interrogation without compromising tag information or location data.
A need, therefore, exists for an RFID system which adaptively changes or controls the interrogation cycle to optimize the time required for accurate exchange of data between the tag/device and reader.